tap: Refactoring macvtap.c

macvtap module has code for tap/queue management and link management. This patch splits
the code into macvtap_main.c for link management and tap.c for tap/queue management.
Functionality in tap.c can be re-used for implementing tap on other virtual interfaces.

Signed-off-by: Sainath Grandhi <sainath.grandhi@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>